1. Field of the Invention
This invention relates to a process for the production of an aromatic urethane. More particularly, it relates to a process for the production of an aromatic urethane by interaction of an aromatic nitro compound, an organic compound having at least one hydroxyl group therein (hereinafter referred to as a hydroxyl group-containing organic compound) and carbon monoxide at elevated temperature under high pressure in the presence of a catalyst.
2. Description of the Prior Art
In most cases, urethanes have been produced by the reaction of isocyanates and hydroxyl group-containing organic compounds. In recent years, there have been developed many novel processes for producing urethanes partly due to shortages and rising costs of starting materials for producing isocyanates and partly due to the strong toxicity of intermediates. However, the newly developed processes still have several problems to be solved and, therefore, are not put into practice for production on an industrial scale.
For example, U.S. Pat. No. 3,338,956 describes a process for producing an aromatic urethane from an alcohol, carbon monoxide and an aromatic nitro compound in the presence of rhodium chlorocarbonyl. However, this process is not economically advantageous in efficiently obtaining highly pure aromatic urethane since the yield is low even though the reaction is effected over a long period of time in the presence of a large quantity of catlyst.
Further, German Pat. No. 1,543,051 teaches a process for producing an aromatic urethane wherein a hydroxyl group-containing organic compound, carbon monoxide and a nitro compound are interacted in the presence of a catalyst of a carbonyl group-containing derivative of a metal of group VIII of the Periodic Table in coexistence with a promoter composed of a salt of a metal selected from metals capable of existing in two or more valence states. However, this process is of little industrial practical value since the yield of product is low even when a mononitro organic compound is used as starting material and becomes lower when a dinitro compound is used.
Further, U.S. Pat. No. 3,531,512 describes a process using a catalyst of palladium and a Lewis acid in which the yield of urethane reaches as high as 80%-90% under certain conditions even when a dinitro compound is employed as starting material. In order to attain such high yield, however, it is essential to effect the urethanation reaction under such severe conditions as a carbon monoxide initial pressure of 190-350 kg/cm.sup.2 and a reaction temperature of 190.degree.-200.degree. C. In addition, the process has a vital disadvantage from an industrial viewpoint in that the Lewis acid, e.g., ferric chloride, used as a promoter materially attacks a metal such as stainless steel by its corrosive action. Accordingly, to put this process into practice on an industrial scale requires use of a glass or tantalum reactor. However, the use of glass or tantalum in such high temperature and high pressure reactions involves several technical and economical disadvantages.
Moreover, there is described in French Pat. No. 2,197,862 a process using as catalyst selenium, sulfur or a compound thereof and a base or water. This process is advantageous in that the reaction conditions are milder than those of other processes which have been known to date and that the yield of urethane is relatively high, but disadvantageous in that amines are secondarily produced. By this process, a urethane with high purity is difficult to obtain since the urethane product is readily contaminated with the secondarily produced amines and the selenium or sulfur used as catalyst.